Natural, pre-existing fractures 99Finally, rotate the tracing paper
back to north, measure the dip dir-
ection of the plane and write it in
front of the dip angle. The orienta-
tion of the plane is therefore found
to be 017/76.(b) Acute and obtuse angles between the boreholes(^298138) Rotate the tracing paper so that
the plane containing BH1 and BH2
lies on the great circle. Measure
along the great circle, using the
small circles to count, to determ-
ine the angle between BH1 and
BH2. It is 629 and as this is less
than 90" it is the acute angle. The
obtuse angle is the complement of
this, i.e. 118". This figure can also
be found by adding together the
angles between BH1 and the up-
per periphery, and BH2 and the
lower periphery.
oi7m
(c) Orientation of a borehole which bisects the acute angle
298f38 055R2
The acute bisector is midway
between BH1 and BH2 along the
great circle representing the plane
017/76. With the tracing paper ro-
tated as shown, mark a cross 31"
from either BH1 or BH2.
Rotate the tracing paper so that
the bisector lies on the E-W line,
mark a tick on the periphery and
write the plunge of the line next to
it.